Apical and basal orthodromic population spikes in hippocampal CA1 in vivo show different origins and patterns of propagation

There is controversy concerning whether orthodromic action potentials origi nate from the apical or basal dendrites of CA1 pyramidal cells in vivo. The participation of the dendrites in the initialization and propagation of po pulation spikes in CA1 of urethananesthetized rats in vivo was studied usin g simultaneously recorded field potentials and current source density (CSD) analysis. CSD analysis revealed that the antidromic population spike, evok ed by stimulation of the alveus, invaded in succession, the axon initial se gment (stratum oriens), cell body and similar to 200 mum of the proximal ap ical dendrites. Excitation of the basal dendrites of CA1, following stimula tion of CA3 stratum oriens, evoked an orthodromic spike that started near t he cell body or initial segment and then propagated similar to 200 mum into the proximal apical dendrites. In contrast, the population spike that foll owed excitation of the apical dendrites of CA1 initiated at the proximal ap ical dendrites, 50-100 mum distal to the cell body layer, and then propagat ed centripetally to the cell body and the proximal basal dendrites. A late apical dendritic spike may arise in the mid-apical dendrites (250-300 mum f rom the cell layer) and propagated distally. The origin or the pattern of p ropagation of each population spike type was similar for near-threshold to supramaximal stimulus intensities. In summary, population spikes following apical dendritic and basal dendritic excitation in vivo appeared to origina te from different locations. Apical dendritic excitation evoked a populatio n spike that initiated in the proximal apical dendrites while basal dendrit ic excitation evoked a spike that started near the initial segment or cell body. An original finding of this study is the propagation of the populatio n spike from basal to apical dendrites in vivo or vice versa. This backprop agation from one dendritic tree to the other may play an important role in the synaptic plasticity among a network of CA3 to CA1 neurons.